DE102015111681B4 - Gas sensor element - Google Patents

Abstract

A gas sensor element (100; 100A) comprising: a detecting portion (10) having a stack of a solid electrolyte body and a heat generating body, said solid electrolyte body having at least a pair of electrodes (4) on opposite sides thereof and said heat generating body including a heat generating source (6) ; anda porous protective layer (20; 20A) formed around said detecting portion (10), said porous protective layer (20; 20A) having a thermal conductivity λ in a range of 0.2 to 5 W / mK and a λCpρ in a range of 5 , 3 × 10 to 2.1 × 10 WJ / mK, where λCpρ is a product of thermal conductivity λ (W / mK), density ρ (g / m) and specific heat Cp (J / gK); and wherein the porous protective layer (20; 20A) further has a capillary radius in a range of 0.01 to 10 μm.

Description

USING A PRIORITY

The present application claims priority to Japanese Patent Applications JP 2014-147482 , filed on 18 July 2014, and JP 2015-091445 , filed on April 28, 2015, the contents of which are incorporated herein by reference.

BACKGROUND

Technical area

The present invention relates to a gas sensor element which is mounted, for example, on a vehicle and detects the oxygen concentration in exhaust gas.

Previous state of the art

Diverse efforts have been made worldwide to reduce environmental impacts and pressures in a variety of industries. In particular, in the automobile industry, development has been promoted day by day not only for fuel-efficient vehicles with gasoline engines, but also for so-called environmentally friendly vehicles such as hybrid vehicles or electric vehicles, and for further improving the performance of such vehicles.

The purification of exhaust gas and the improvement of the fuel efficiency of vehicles are performed by detecting the oxygen concentration in a measurement target gas, such as exhaust gas, using a gas sensor, and precisely controlling the fuel consumption and the intake air amount.

An exemplary basic configuration of a gas sensor element constituting such a gas sensor includes a detecting portion having a stack of a solid electrolyte body having a pair of electrodes on opposite sides thereof and a heat generating body including a heat generating body and a porous protective layer formed around the detecting portion ,

A gas sensor detects the oxygen concentration in exhaust gas at a high temperature of about 400 to 850 ° C. Thus, when water droplets (ie, condensed water) in the exhaust gas collide with the gas sensor element forming the gas sensor, there is a fear that thermal shock may occur due to partial cooling, and abnormal output may be generated due to a volume change of the element with a temperature change can, which results from a drop in temperature when the element is wet, or that an abnormal output can be generated, resulting from cracking of the element due to the thermal shock. Further, it is also feared that metallic compounds in the condensed water may penetrate into the element together with the water, which in turn may poison the detecting portion of the gas sensor element.

In order to eliminate such concerns, a porous protective layer is provided around the detecting portion of the gas sensor element.

Herein, Patent Document 1 as a conventional art relating to a gas sensor element having a porous protective layer discloses a gas sensor element in which the periphery of the element is surrounded by a porous protective layer of alumina to prevent the collision of water droplets. Moreover, Patent Document 2 discloses a gas sensor element having a porous protective layer made of a single silicon carbide or aluminum nitride material or a material which is a mixture thereof with other ceramic materials. Further, Patent Document 3 discloses a gas sensor element having on an outer gas introduction surface of a diffusive resist layer of the main body portion of the element through which a measurement target gas is introduced, a porous protective layer for trapping toxic components in the measurement target gas, and a surface protective layer formed on the porous protective layer a high temperature at which a solid electrolyte body becomes active, a water-repellent property and has a lower porosity than the porous protective layer.

As described above, a variety of improvements have been made on a porous protective layer provided around a detecting portion to improve the water resistance of gas sensor elements. In particular, technology has been developed primarily with respect to a material that is applied to the porous protective layer. It should be noted that in Patent Document 3, the surface roughness of the surface protective layer having a lower porosity than the porous protective layer is defined to prevent cracking of the element which would otherwise occur due to thermal shock when the element becomes wet, thereby the water-repellent property of the surface protection layer can be ensured.

Other patent and non-patent documents worth mentioning include the following: Patent Document 4 relates to a gas sensor element having a protective layer which is required to be very durable. Patent Document 5 relates to a gas sensor element having a sensor section, a heater section, and a porous protective layer. Patent Document 6 relates to a gas sensor element having a detection area comprising a solid electrolyte body. Patent Document 7 relates to a ceramic heater for heating a gas sensor element. Non-patent document 1 relates to technical ceramics of the company Kyocera.

By configuring a porous protective layer of a gas sensor element to repel condensed water in exhaust gas, it becomes possible to significantly reduce a thermal shock that can be generated in the porous protective layer or the gas sensor element and also to inhibit the penetration of the condensed water and thereby to solve the problem of possible poisoning of the detection section.

With respect to the water-repellent property of the porous protective layer, the inventors have found that a porous protective layer having an excellent water-repellent property can be obtained by a different approach than that of the conventional art.

DOCUMENTS OF THE RELATED ART

Patent documents

• Patent Document 1: JP 2009-80110 A
• Patent Document 2: JP 2011-237222 A
• Patent Document 3: JP 2012-93330 A
• Patent Document 4: JP 2011-252894 A
• Patent Document 5: US 2010/0155240 A1
• Patent Document 6: US 2014/0291150 A1
• Patent Document 7: DE 10308558 A1
• Non-Patent Document 1: "Characteristics of Kyocera Technical Ceramics". Datasheet [online]. Kyocera Corporation, July 2012
• http://web.archive.org/web/20121115045330/http://global.kyocera.com/prdct/fc/product/pdf/material.pdf>.

SHORT VERSION

The present invention has been made in view of the above problems, and an object of the present invention is to provide a gas sensor element having a porous protective layer which has a superior water repellency.

In order to achieve the above object, a gas sensor element in accordance with the present invention includes a detection section having a stack of a solid electrolyte body having at least a pair of electrodes on opposite sides thereof and a heat generating body including a heat generating body and a porous protective layer surrounding the detection section is formed. The porous protective layer has a thermal conductivity λ in the range of 0.2 to 5 W / mK and a λCpρ, which is the product of thermal conductivity λ (W / mK), density ρ (g / m ³ ) and specific heat Cp (J / gK) is in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² .

In the gas sensor element in accordance with the present invention, two physical values, namely the thermal conductivity and the λCpρ value (λ: thermal conductivity (W / mK), p: density (g / m ³ ) and Cp: specific heat (J / gK) )) of the porous protective layer in the foreground, and the numerical ranges of such values are defined so as to ensure a more excellent water-repellent property of the porous protective layer.

The thermal conductivity of the porous protective layer determines the rate of transfer of heat from the porous protective layer to condensed water, and the rate determines the ease with which a vapor film is formed. The fact that a vapor film can be easily formed means that the porous protective layer has an excellent water-repellent property. In addition to the ease with which a vapor film is formed, the amount of heat necessary to form a vapor film is also an important factor. That is, the porous protective layer can have an excellent water-repellent property only if it has a physical value capable of generating an amount of heat necessary for forming a vapor film.

The inventors have found that the water-repellent performance of the porous protective layer can be determined with high accuracy when the physical value corresponding to the heat quantity is represented by the product of thermal conductivity, density and specific heat.

The water-repellent property of the porous protective layer is obtained by a film boiling phenomenon (i.e., Leidenfrost phenomenon). The Leidenfrost phenomenon is a phenomenon whereby the surface of a water droplet, when it contacts the surface of the porous protective layer at a high temperature, instantly evaporates and the resulting vapor forms a shielding layer between the surface of the porous protective layer and the water droplet. Due to the Leidenfrost phenomenon, even if a water droplet adheres to the surface of the porous protective layer, the water droplet is immediately separated from the surface of the porous protective layer. This means that there is a water-repellent property.

The thermal conductivity λ is defined in the range of 0.2 to 5 W / mK. In this case, when the thermal conductivity λ is in the range of greater than or equal to 0.2 W / mK, a film boiling phenomenon occurs, and thus a water-repellent property is present. Thus, the lower limit of the thermal conductivity is defined as 0.2 W / mK. In addition to the water-repellent property, the response properties are further important properties of the gas sensor element. From the aspect of the response characteristics, the upper limit of the thermal conductivity is defined as 5 W / mK. It is known that there is a correlation between the thermal conductivity and the porosity of a porous protective layer, and there is also a correlation between the response characteristics of a gas sensor element and the porosity of a porous protective layer.

The inventors have found that an excellent water repellency is obtained when the heat conductivity of the porous protective layer is in the aforementioned numerical range at a temperature in the range of 400 to 850 ° C in which the gas sensor is controlled.

Meanwhile, the λCpρ value is defined in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² . Here, at a temperature of less than or equal to 850 ° C exposed to the gas sensor element, a film boiling phenomenon occurs, and a water repellency is present when the λ Cpρ value is in the range of greater than or equal to 5.3 × 10 ⁵ WJ / m ⁴ K ^2. Thus, the lower limit of the λCpρ value is defined as 5.3 × 10 ⁵ WJ / m ⁴ K ² . Meanwhile, the upper limit of the λCpρ value is defined as 2.1 × 10 ⁷ WJ / m ⁴ K ² from the aspect of the response characteristics.

According to the gas sensor element of the present invention, by defining the two physical values, thermal conductivity and λCpρ value (λ: thermal conductivity (W / mK), p: density (g / m ³ ) and Cp: specific heat (J / gK)) of the porous ones Protective layer can be provided in predetermined numerical ranges, a gas sensor element having an excellent water-repellent property and excellent response characteristics.

The inventors have determined that the thermal conductivity λ in the range of 0.2 to 5 W / mK corresponds to the porosity in the range of 10 to 50% of the porous protective layer. Further, the inventors also found that the λCpρ value in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² corresponds to the porosity in the range of 10 to 50% of the porous protective layer. It should be noted that the porosity of the porous protective layer is desirably set particularly in the range of about 30% or the range of 20 to 50% of the aforementioned numerical range.

Further, in the gas sensor element of the present invention, the porous protective layer has a capillary radius in the range of 0.01 to 10 μm.

As a result of checking the relationship between the capillary radius and the gas response time of the porous protective layer, a result has been obtained such that the gas response time becomes shorter with larger capillary radius, and saturation of the gas response time occurs at the shortest time when the capillary radius is 0.01 μm. Based on such experimental results, the lower limit of the capillary radius is defined as 0.01 μm.

Meanwhile, as a result of checking the relationship between the capillary radius of the porous protective layer and the penetration of water droplets, the inventors have verified that the penetration distance with larger capillary radius becomes longer, and that the penetration distance is greater than or equal to 700 μm, which is currently considered to be the maximum value of thickness porous protective layer is considered at a capillary radius of 10 microns. Thus, the upper limit of the capillary radius is defined as 10 μm based on the experimental results.

In the present case, the aforementioned porous protective layer may have any of the following two structures.

The first structure is a single-layered structure formed of an alumina-containing aggregate and a siliceous coating material. A series of alumina-containing aggregates are bonded by silica, which is a binder, thereby forming a porous protective layer having a single-layered structure.

Meanwhile, the second structure is a stacked structure of a lower layer which is in contact with the detection section located in the gas sensor element, and an upper layer which is in contact with the exterior of the gas sensor element. The porosity of the upper layer is comparatively lower than that of the lower layer. At least the upper layer has a two-layered structure formed of an alumina-containing aggregate and a siliceous coating material.

When the upper layer to be in contact with condensed water is formed by using an alumina-containing aggregate and a siliceous coating material so that the upper layer becomes comparatively denser than the lower layer, the upper layer may surely have a water-repellent property and The lower layer may certainly have the property of trapping toxic substances, since the lower layer becomes more porous and has a larger specific surface area than the upper layer.

As understood from the above description, according to the gas sensor element of the present invention, by defining the two physical values: thermal conductivity and λCpρ value (λ: thermal conductivity (W / mK), p: density (g / m ³ ) and Cp: specific Heat (J / gK)) of the porous protective layer in predetermined numerical ranges, a gas sensor element having an excellent water-repellent property and excellent response characteristics can be provided.

Thus, the operation and advantageous effects of claim 1 are that by defining the product of thermal conductivity, density and specific heat, a shielding film can be formed between the surface of the porous protective film and a water droplet, thus providing a more excellent water-repellent property of the porous protective film can. The operation and advantageous effects of claim 2 are that by defining the capillary radius of the porous protective film, saturation of the gas response time is allowed and the penetration distance of water droplets can be maximized. The operation and advantageous effects of claim 3 are that by defining the porosity of the porous protective film, the response characteristics and the thermal conductivity of the gas sensor element can be optimized. The operation and advantageous effects of claim 4 are that by coating an aggregate having high fire resistance and high hardness with a coating material using a sol-gel method, a porous protective film having a large specific surface area can be formed. The operation and advantageous effects of claim 5 are that the upper layer to be in contact with condensed water is formed using an alumina-containing aggregate and a siliceous coating material so that the upper layer becomes comparatively denser than the lower layer. whereby the upper layer sure a water-repellent property and the lower layer may surely have the property of trapping poisonous substances because the lower layer becomes more porous and has a larger specific surface area than the upper layer.

list of figures

• 1 is a schematic representation of the embodiment 1 of a gas sensor element of the present invention.
• 2 is a schematic representation of the embodiment 2 of a gas sensor element of the present invention.
• 3 Fig. 14 is a graph showing experimental results indicating the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 μ1 and satisfying a water repellency amount of 3 μ1 and the heat conductivity of a porous protective layer.
• 4 Fig. 12 is a graph showing experimental results indicating the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 μ1 and the λCpρ value of a porous protective layer.
• 5 Figure 11 is a graph showing experimental results indicating the relationship between porosity, thermal conductivity and response characteristics.
• 6 Fig. 12 is a graph showing experimental results indicating the relationship between the porosity, the λCpρ value and the response characteristics.
• 7 Figure 11 is a graph showing experimental results indicating the relationship between the capillary radius, the gas response time, and the penetration distance.

DETAILED DESCRIPTION OF THE EMBODIMENT (S)

Hereinafter, embodiments of a gas sensor element of the present invention will be described with reference to the drawings.

Embodiment 1 of a gas sensor element

1 is a schematic representation of the embodiment 1 of a gas sensor element of the present invention. An in 1 shown gas sensor element 100 generally includes a detection section 10 , which detects the oxygen concentration in exhaust gas, and a porous protective layer 20 representing the scope of the acquisition section 10 Protects against moisture in the exhaust and thus prevents abnormal outputs, which otherwise due to a temperature drop of the detection section 10 which results from moistening the detection section 10 resulted, and which also intercepts hydrogen gas, carbon monoxide gas and the like, which passes through it.

The detection section 10 generally includes a solid electrolyte layer 3 which on opposite sides thereof a pair of electrodes 4 comprising an electrode 41 on the measurement target gas side and an electrode 42 on the reference gas side, a porous, diffusive resistance layer 2 that the electrode 41 on the measurement target gas side over a measurement target gas space 8th surrounds, a shielding layer 1 that the measurement target gas space 8th together with the porous, diffusive resistance layer 2 limited, a reference gas space protective layer 5 that the electrode 42 on the reference gas side via a reference gas space 9 surrounds, and a heat generation source 6 and a heat generation source substrate 7 ,

The heat generation source 6 includes a heater that is a heat generating body, and forms a heating area of the gas sensor element 100 so that it is thermally controlled to reach its activation temperature.

The detection section 10 has in the cross-sectional shape shown in the drawing, corner portions which are recessed in a tapered shape. With such recessed portions, the thickness of the porous protective layer becomes 20 at the corresponding sections of the acquisition section 10 ensured.

The solid electrolyte layer 3 is made of zirconia, and the electrode 41 on the target gas side and the electrode 42 on the reference gas side are each made of platinum. Furthermore have the shielding layer 1 and the reference gas space protective layer 5 each have a gas-impermeable inner structure and are formed of alumina.

A voltage in which there is a linear correlation between the oxygen concentration difference and the current becomes above the pair of electrodes 4 applied, and a measurement target gas is with the electrode 41 on the measuring target gas side, while a reference gas, such as air, with the electrode 42 on the reference gas side is brought into contact. Then, the value of the current generated between the electrodes in accordance with each oxygen concentration difference is measured, so that the air-fuel ratio of the vehicle engine can be identified based on the measured current.

The porous, diffusive resistance layer 2 is provided at a position that the measurement target gas space 8th around the electrode 41 bounded on the measurement target gas side to the amount of measurement target gas entering the electrode 41 is introduced to keep down on the measurement target gas side, and is configured to further hydrogen gas, carbon monoxide gas, and the like of the exhaust gas, via the porous protective layer 20 around the detection section 10 were introduced into the measurement target gas space 8th via the porous, diffusive resistance layer 2 contribute.

The porous protective layer 20 is a porous layer containing alumina on the surface of which noble metal catalyst particles (not shown) are supported, and silica sol. The noble metal catalyst particles in the porous protective layer 20 can cover the entire area of the porous protective layer 20 be distributed or can only in the side area of the same, that of the porous, diffusive resistance layer 2 near the electrode 41 on the measurement target gas side, be distributed. Alternatively, the amount of noble metal catalyst particles present in the porous protective layer 20 are supported, distributed such that a relatively larger amount of noble metal catalyst particles is supported in a range, for example, the porous, diffusive resistance layer 2 equivalent. In the present case, as the noble metal catalyst particles, palladium or rhodium alone may be used, or an alloy containing two or more of palladium, rhodium and platinum may be used.

The porous protective layer 20 can be made by using a so-called dipping method, the repeated operations of immersing the detecting portion 10 in a slurry containing an alumina, spinel, silicon carbide and / or aluminum nitride-containing aggregate and a silica, alumina, titania, zirconia, antimony oxide and / or zinc oxide-containing coating material, and taking out the detection portion 10 from the slurry and drying it until a predetermined thickness is obtained. Alternatively, the porous protective layer 20 be prepared using a flame spraying process, which is suitable for the production of a dense layer. If, as regards alumina, particles containing α-alumina, γ-alumina, O-alumina and / or spinel are applied, it is possible to apply the porous protective layer 20 to form with a large specific surface and thereby improve the property of trapping toxic substances. In particular, for the preparation of the porous protective layer 20 Preferably, a slurry containing an alumina-containing aggregate and a silica-containing coating material is used.

For the porous protective layer 20 For example, the thermal conductivity λ is set in the range of 0.2 to 5 W / mK, and further, λCpρ is the product of thermal conductivity λ (W / mK), density ρ (g / m ³ ) and specific heat Cp (J / gK ) is set in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² .

If the thermal conductivity λ and the λCpρ value are set in the aforementioned numerical ranges, it is possible, due to the Leidenfrost phenomenon, particularly in the temperature range of 400 to 850 ° C, in which the gas sensor element 100 is controlled to provide excellent water repellency and excellent response characteristics.

The capillary radius of the porous protective layer 20 is set in the range of 0.01 to 10 μm.

When the capillary radius of the porous protective layer 20 is set in the aforesaid numerical range, the gas response time is saturated at about 300 ms, which is the shortest time especially at a capillary radius of 0.01 μm. Furthermore, with respect to the penetration distance of the condensed water, the penetration distance is 700 μm, which is currently regarded as the maximum value of the thickness of the porous protective layer, especially with a capillary radius of 10 μm. Thus, if the capillary radius of the porous protective layer 20 is set in the range of 0.01 to 10 μm, the response characteristics and the penetration distance of water droplets can be controlled within optimal ranges.

Embodiment 2 of the gas sensor element

2 is a schematic representation of the embodiment 2 of a gas sensor element in accordance with the present invention. An in 2 shown gas sensor element 100A includes a porous protective layer 20A with a two-layer stacked structure of a lower layer 20ab that with the capturing section 10 in contact, and an upper layer 20aa that is in contact with the exterior.

For the porous protective layer 20A is the porosity of the upper layer 20aa comparatively lower than that of the lower layer 20ab , and at least the top layer 20aa is formed using an aggregate containing alumina, spinel, silicon carbide and / or aluminum nitride and a coating material containing silica, alumina, titania, zirconia, antimony oxide and / or zinc oxide. Will the upper layer 20aa which is in direct contact with condensed water, using an aggregate containing alumina and silica sol, and as a comparatively denser layer than the lower layer 20ab formed, so can the upper layer 20aa certainly have a water-repellent property, and the lower layer 20ab can certainly have the property of trapping toxic substances as the lower layer 20ab becomes more porous and has a larger specific surface than the top layer 20aa , In particular, for the preparation of the porous protective layer 20aa Preferably, a slurry containing an alumina-containing aggregate and a silica-containing coating material is used.

If the porous protective layer 20A the thermal conductivity λ is set in the range of 0.2 to 5 W / mK and λCpρ, which is the product of thermal conductivity λ (W / mK), density ρ (g / m ³ ) and specific heat Cp (J / gK), is set in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² , it is also possible to use the gas sensor element 100A to provide excellent water repellency and excellent response properties.

Experiments for determining the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 .mu.l and the thermal conductivity of a porous protective layer, experiments for determining the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 .mu.l , and the λCpρ value of a porous protective layer, and their results

The inventors have made experiments for determining the relationship between the ambient temperature and the thermal conductivity and the λCpρ value of a porous protective layer, which are two physical values ranging from 400 to 850 ° C in which a gas sensor is controlled an excellent water repellency required by the porous protective layer against a water droplet of 3 μl.

In the present case, the porous protective layer has a single-layer structure, which in 1 is shown. A slurry was prepared by dispersing 8 μm of α-alumina, 20% by mass of silica sol and a dispersing agent (PVA) in water, and the slurry was used for a range in which the porosity is over 30% by using a peripheral dipping method Detecting portion of the gas sensor element made to adhere, and then a two-hour firing in the atmosphere at 900 ° C was performed to produce a porous protective layer, while for a range in which the porosity is in the range of less than or equal to 30%, a porous protective layer with similar specifications using a flame spraying process.

3 Fig. 12 is a graph showing experimental results indicating the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 μl and the thermal conductivity of a porous protective layer. 4 Fig. 12 is a graph showing experimental results indicating the relationship between the ambient temperature of a gas sensor capable of rejecting an amount of water of 3 μl and the λCpρ value of a porous protective layer.

3 can confirm that the thermal conductivity in the form of a quadratic function curve decreases with respect to the ambient temperature, and the thermal conductivity at a temperature of 850 ° C is 0.2 W / mK, while the thermal conductivity at a temperature of about 500 ° C becomes 5W / mK is.

Indes may 4 confirm that the λCpρ value also decreases in the form of a quadratic function curve with respect to the ambient temperature, and the λCpρ value at a temperature of 850 ° C decreases 5.3 × 10 ⁵ WJ / m ⁴ K ² , while the λCpρ value at a temperature of about 500 ° C (ie, a temperature equal to or greater than 400 ° C) is 2.1 × 10 ⁷ WJ / m ⁴ K ² .

Experiments to determine the relationship between porosity, thermal conductivity and response properties, experiments to determine the relationship between porosity, λCpp value and response properties, and their results

The inventors have made experiments for determining the relationship between the respective porosity, thermal conductivity and response characteristics and the relationship between the respective porosity, the λCpρ value and the response characteristics by changing the porosity. It should be noted that the method for producing a porous protective layer and the use thereof are the same as those in the aforementioned experiments.

5 Figure 11 is a graph showing experimental results indicating the relationship between porosity, thermal conductivity and response characteristics. 6 Fig. 12 is a graph showing experimental results indicating the relationship between porosity, λ Cpp value and response characteristics.

5 can confirm that the thermal conductivity at a porosity of 50% is 0.2 W / mK, while the thermal conductivity at a porosity of 10% is 5 W / mK.

Out 5 It can be seen that the thermal conductivity is desirably not greater than 5 W / mK, since the response characteristics become too strong when the porosity is in the range of less than 10% (ie, when the thermal conductivity is in the range of over 5 W / mK).

In the 3 and 5 The experimental results shown can confirm that a preferable range of the thermal conductivity of the porous protective layer can be defined in terms of the water repellency and the response characteristics in the range of 0.2 to 5 W / mK.

Indes may 6 confirm that the λCpρ value at a porosity of 50% is 5.3 × 10 ⁵ WJ / m ⁴ K ² , while the λCpρ value at a porosity of 10% is 2.1 × 10 ⁷ WJ / m ⁴ K ² is.

Out 6 It can be seen that the λCpp value is desirably not larger than 2.1 × 10 ⁷ WJ / m ⁴ K ² , since the response characteristics become too strong when the porosity is in the range of less than 10% (ie, the λCpρ value is in the range of over 2.1 × 10 ⁷ WJ / m ⁴ K ² ).

The in the 4 and 6 The experimental results shown can confirm that a preferable range of the λCpρ of the porous protective layer is defined to be in the range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² from the aspect of the water repellency and the response properties can.

Experiments on the relationship between the ratio of the cross section of a water droplet to the water repellency, and results thereof

The inventors have made experiments for determining the relationship between the ratio of the cross section of a water droplet and the water repellency. The experiments verify the relationship between the contact area of a solid liquid on the porous protective layer which, when a film boiling phenomenon occurs when a water droplet is dropped in an amount of 3 μl, has a water repellency required by the porous protective layer and the cross section of a water droplet. Table 1 below shows the results of the verification. Table 1 Ratio of the cross section of a water droplet Water-repellent property 0.01 ○ 0.2 ○ 1 ○ 2 ◯ 4 ◯ 5 × 7 × Notes: "◯" indicates that there is a phenomenon of water repellency on the entire surface of the porous protective layer, while "×" indicates that no water repellency phenomenon occurs on parts of the surface of the porous protective layer.

It is noted from Table 1 that when the ratio of the cross section of a water droplet becomes five times or more, the temperature of the surface of the porous protective layer decreases when a vapor film is formed due to an increased contact area of the solid liquid, so that the formation of a vapor film is difficult on parts of the layer and thus such parts have no phenomenon of water repellency.

Experiments to change the temperature of the surface of a porous protective layer and the water-repellent property, and results thereof

The inventors have made experiments for determining the relationship between a temperature change of the surface of a porous protective layer and the water repellency. The experiments verify the degree of temperature change of the surface of a porous protective layer which has a water repellency required by the porous protective layer when a film boiling phenomenon occurs when a water droplet having a drop amount of 3 μl is dropped. Table 2 below shows the results of the verification. Table 2 Temperature change of the surface of the porous protective layer Water-repellent property 100 ◯ 130 ◯ 150 ◯ 200 × Notes: "◯" indicates that there is a phenomenon of water repellency on the entire surface of the porous protective layer, while "×" indicates that no water repellency phenomenon occurs on parts of the surface of the porous protective layer.

It is noted from Table 2 that the formation of a vapor film or the maintenance of the vapor film becomes difficult when the temperature change of the surface of the porous protective layer exceeds 150 ° C (when the amount of temperature change is 150 ° C or more). Thus, there is no water-repellent property.

Experiments to determine the relationship between capillary radius, gas response time and penetration distance, and results thereof

The inventors have also made experiments for determining the relationship between capillary radius, gas response time and penetration distance. Herein, the capillary radius was defined based on the result of a measurement made by segregating a part of a porous protective layer and measuring the pore distribution using a mercury porosimeter.

As a method of judging the gas response characteristics, a sensor output obtained by switching gas between A / F13 (rich side) and A / F18 (lean side) for an example of a gas sensor element having a porous protective layer was used, and 63% of the response time the obtained sensor output was evaluated. In this case, N _2, C ₃ H ₈ and O ₂ were used as the base gas, and O ₂ was used as the injection gas.

Meanwhile, as a method of judging the penetration, 0.3 μl of a water-simulating aqueous ferric chloride solution was discharged onto a specimen using a dispenser, and the specimen was cast in resin. Then, the cross section of the porous protective layer was observed to measure the penetration distance. 7 shows the results of the measurement.

7 can confirm that the gas response time decreases in a curved manner with increasing capillary radius and the gas response time is saturated at about 300 ms, which is the shortest time when the capillary radius is 0.01 μm. The infiltration distance of condensed water increases in a curved manner with increasing capillary radius, and the penetration distance is 700 μm, which is currently regarded as the maximum value of the thickness of the porous protective layer at a capillary radius of 10 μm. From such results, it is noted that the response characteristics and the penetration distance of water droplets can be controlled within optimum ranges when the capillary radius of the porous protective layer is set in the range of 0.01 to 10 μm.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the specific embodiments are not limited thereto. The present invention includes any design changes and the like that are possible within the spirit and scope of the present invention.

LIST OF REFERENCE NUMBERS

1

Shielding layer

2

Porous, diffusive resistance layer

3

Solid electrolyte layer

4

electrode pair

41

Electrode on the target gas side

42

Electrode on the reference gas side

5

Reference gas space protective layer

6

Heat source (heater)

7

Heat generation source substrate

8th

Measurement target gas chamber

9

Reference gas chamber

10

detecting section

20,20A

Porous protective layer

20aa

Upper layer

20ab

Lower class

100,100A

Gas sensor element

Claims (4)

A gas sensor element (100; 100A) comprising: a detecting portion (10) having a stack of a solid electrolyte body and a heat generating body, said solid electrolyte body having at least a pair of electrodes (4) on opposite sides thereof and said heat generating body including a heat generating source (6) ; and a porous protective layer (20; 20A) formed around said detecting portion (10), said porous protective layer (20; 20A) having a thermal conductivity λ in a range of 0.2 to 5 W / mK and a λCpρ in a range of 5.3 × 10 ⁵ to 2.1 × 10 ⁷ WJ / m ⁴ K ² , where λCpρ is a product of thermal conductivity λ (W / mK), density ρ (g / m ³ ) and specific heat Cp (J / gK); and wherein the porous protective layer (20; 20A) further has a capillary radius in a range of 0.01 to 10 μm. Gas sensor element after Claim 1 wherein the porous protective layer (20; 20A) has a porosity in a range of 10 to 50%. Gas sensor element after Claim 1 wherein the porous protective layer (20; 20A) has a single-layer structure comprising an aggregate and a coating material, the aggregate containing alumina, silicon carbide, spinel and / or aluminum nitride and the coating material silica, alumina, titania, zirconia, antimony oxide and / or or zinc oxide. Gas sensor element after Claim 1 wherein the porous protective layer (20; 20A) has a stacked structure of a lower layer (20Ab) and an upper layer (20Aa), the lower layer (20Ab) being connected to the detecting section (10A) in the gas sensor element (100; ), and the upper layer (20Aa) faces an outer side of the gas sensor element (100; 100A), a porosity of the upper layer (20Aa) is comparatively lower than a porosity of the lower layer (20Ab), and at least the upper layer (20Aa ) has a two-layered structure including an aggregate and a coating material, the aggregate containing alumina, silicon carbide, spinel and / or aluminum nitride and the coating material comprises silica, alumina, titania, zirconia, antimony oxide and / or zinc oxide.

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